1. Field
The present disclosure relates to an attachable image display device that includes a light guiding prism, and an ocular optical system that includes a light guiding prism.
2. Background
Small attachable image display devices that are used attached to, for example, glasses have attracted attention in recent years. An attachable image display device includes, for example, a display element and an ocular optical system including a light guide prism, and it guides image light from the display element and emits the image light to an eye of a user, so as to display a display image of the display element in a field of view of a user.
Light guiding prisms have been made smaller and thinner because they are provided over a field of view of a user. However, unnecessary reflected light is created in a prism due to the prism being made smaller and thinner, which may result in forming, in the eye of the user, an image created with light other than image light that is to be originally generated (hereinafter referred to as a ghost image).
FIG. 1 is a perspective view of examples of a display element and an ocular optical system that are included in a conventional attachable image display device. FIG. 2 is a perspective view of the ocular optical system as viewed from a different direction than that of FIG. 1. FIG. 3 is a diagram that illustrates an example of a beam that is emitted from the center of a display area of the display element and passes through cross sections of the ocular optical system.
In FIGS. 1 to 3, an ocular optical system 1 includes a light guiding prism 3 that guides image light from a display element 2, and an eyepiece 4 that emits the image light guided by the light guiding prism 3 to an eye of a user.
The light guiding prism 3 includes four sides 3a, 3b, 3c, and 3d that are arranged to surround a light path of the image light from the display element 2, and a reflection surface 3e off which the image light from the display element 2 is reflected to the eyepiece 4.
The eyepiece 4 includes an emission surface 4a that has a positive refractive power, and the emission surface 4a emits the image light reflected off the reflection surface 3e. 
An optical axis 5 (5a and 5b) is a beam emitted from the center of a display area of the display element 2 to pass through the center of the emission surface 4a of the eyepiece 4 without being reflected off any surface other than the reflection surface 3e. It is assumed that, in the beam, a portion of the beam before the beam is reflected off the reflection surface 3e is the optical axis 5a and a portion of the beam after the beam is reflected off the reflection surface 3e is the optical axis 5b. 
The sides 3a and 3b of the light guiding prism 3 are surfaces arranged parallel to a plane including the optical axes 5a and 5b. The sides 3c and 3d of the light guiding prism 3 are surfaces that are parallel to the optical axis 5a and perpendicular to the plane including the optical axes 5a and 5b. 
A cross section 3f of the light guiding prism 3 is a cross section formed by a line of intersection of the light guiding prism 3 and a plane that includes the optical axis 5a and that is perpendicular to the plane including the optical axes 5a and 5b. A cross section 3g of the light guiding prism 3 is a cross section formed by a line of intersection of the light guiding prism 3 and a plane that is perpendicular to the optical axis 5a and that is situated closer to the display element 2 than the eyepiece 4. A cross section 3h of the light guiding prism 3 and a cross section 4b of the eyepiece 4 are cross sections formed by a line of intersection of the light guiding prism 3 and a plane that includes the optical axis 5b and that is perpendicular to the plane including the optical axes 5a and 5b. The cross sections 3f, 3g and 3h each have a rectangular or square shape. The cross sections 3f, 3g, 16h, and 4b are also cross sections of the ocular optical system 1.
For example, as illustrated in FIG. 3, when a beam is emitted from the center of the display area of the display element 2 and passes through each of the cross sections 3f, 3h, and 4b of the ocular optical system 1, the ocular optical system 1 having the configuration described above may cause the beam reflected off the side 3a (or the side 3b) of the light guiding prism 3 to enter a pupil of the user (a pupil of an eye 6). In this case, a ghost image is generated in the field of view of the user due to the beam. In particular, an area situated between the reflection surface 3e and the eyepiece 4 at the side 3a (or the side 3b) of the light guiding prism 3 is close to the eyepiece 4, so light reflected off the area will generate a stronger ghost image.
Such a ghost image is generated more easily due to the light guiding prism 3 being made smaller and thinner.
Thus, in an attachable image display device that includes an ocular optical system including a small and thin light guiding prism, a ghost image generated due to light reflected off a side of the light guiding prism is desired to be removed or reduced.